Study On Ultra-low Mercury Catalyst For Acetylene Hydrochlorination

As one of the five general-purpose plastics,polyvinyl chloride（PVC）is widely used in medical,living and construction fields.In view of China’s special energy structure of"rich in coal,poor in oil,and less in gas",the domestic production of vinyl chloride monomer mainly relies on the production of acetylene hydrochlorination.In this process,the traditional activated carbon-supported mercuric chloride（HgCl₂）is still used as the main catalyst,while the highly toxic and volatile HgCl₂not only causes rapid deactivation of the catalyst during production and use,but also brings harm to the environment and humans.Therefore,finding green and efficient catalysts has become an urgent problem to be solved in the PVC industry.Although some progress has been made in mercury-free catalysts in recent years,the high cost,low stability and complex process have limited their large-scale use in industry.At the current stage when mercury-free catalysts are not yet mature,the research on ultra-low mercury catalysts plays a key role in the sustainable development of the PVC industry from low-mercury to mercury-free stages.In this paper,2 wt%HgCl₂of activated carbon was selected as the main active component,and an ultra-low mercury catalyst with high catalytic activity was prepared by pretreatment of the carrier and addition of auxiliary.The details are as follows:（1）Activated carbon was used as the carrier,and a small amount of modifier E was added to pretreat the carrier.On this basis,2 wt%HgCl₂of the activated carbon was loaded as the main active component,and Mt¹Cl₂,Mt²Cl₂and Mt³Cl were used as auxiliary to prepare quaternary ultra-low mercury catalysts by a simple over-volume impregnation method.After formula optimization,the mass ratio of different components in the quaternary ultra-low mercury catalyst with the best catalytic performance is:HgCl₂:Mt¹Cl₂:Mt²Cl₂:Mt³Cl=2:3.1:25:3;In addition,the process conditions of the quaternary ultra-low mercury catalyst were explored.The acetylene conversion rate of the quaternary ultra-low mercury catalyst reached99.72%,and the vinyl chloride selectivity was 99.45%under the optimal reaction conditions 130 ℃,30 h^-1;Finally,stability testing of quaternary ultra-low mercury catalysts was performed at 160 ℃ and 600 h^-1.The study proved that the quaternary ultra-low mercury catalyst exhibited excellent stability within 24 h.EDS,XRD,Tg,BET and other test methods were used to explore the reason for the deactivation of the quaternary ultra-low mercury catalyst.The analysis showed that the pore blockage caused by the deposition of carbon species was proved to be the main reason for the decrease of the catalytic activity of the catalyst.（2）Binary ultra-low mercury catalysts were prepared with Mt¹Cl₂and Mt²Cl₂as auxiliary respectively to explore the interaction between the auxiliary and HgCl₂and its effect on the catalytic performance of the catalyst.The performance evaluation shows that there is a synergistic catalytic effect between the auxiliary Mt¹Cl₂,Mt²Cl₂and mercuric chloride,and the synergistic catalytic effect of Mt²Cl₂is the best.In addition,a series of test methods were used to explore the effect of the addition of auxiliary on the thermal stability and chemical environment of the binary ultra-low mercury catalyst.The study found that the addition of auxiliary changed the chemical environment of the activated carbon surface,thus affecting the catalytic activity and thermal stability of the binary ultra-low mercury catalyst.Comparison of XPS test data of binary ultra-low mercury catalysts before and after reaction,it was found that the addition of Mt¹Cl₂and Mt²Cl₂as auxiliary played a positive role in the stabilization of the active species Hg²⁺.